1. Field
The present invention relates to passive ranging systems and, more particularly, to such systems in which angular position information is the only data obtained from the target.
2. Prior Art
Radar surveillance aircraft are often faced with the problem of operating in a dense and moving environment. The radar displays aboard the surveillance aircraft are adversely affected by jamming sources, but may be restored by locating these jamming sources. It is preferable to accomplish the location of the jamming sources passively from the surveillance aircraft to reduce the probability of detection and to avoid the need for cooperating aircraft or ground stations in locating the jammers.
A number of triangulation techniques are available; however, surveillance aircraft generally do not provide sufficient distance between direction finding antennas to make use of these techniques.
A first prior art nontriangulation technique uses the fact that the intercepted power from a target vehicle is proportional to the inverse of the range squared. Hence, two surveillance aircraft carrying intercept receivers separated by a substantial distance along the line directed at the jammer will measure slightly different intercepted powers which may be used (environment permitting) to calculate range. However, the required separation in receivers does not permit this technique to be carried out with a single aircraft.
A second prior art technique uses the elevation angle of the intercept and an assumed value of the jammer altitude to calculate range. This technique, like the first, is very sensitive to propagation and equipment errors and is based on an altitude assumption which greatly reduces it reliability.
A third scheme is similar to the second, except that the jammer ground reflection is used to obtain an estimate of elevation angle. The time-difference or phase-difference-of-arrival of the direct and ground bounce intercepted signals is computed, and this information (converted to jammer elevation angle) together with the target altitude assumption, is used to calculate range. The continued use of the altitutde assumption again makes this scheme unreliable.
A fourth technique is a further variant which uses the bistatic radar reflection off a nonradiating target caused by a cooperative or noncooperative ground surveillance radar. The time-difference-of-arrival between the direct surveillance radar intercept and the bistatic reflection from a target places the target on an ellipsoid of which the surveillance radar transmitter and the intercept receiver are foci. If the target-bearing angle can be determined using the intercept antenna, the target may then be located on a line which is the intersection of the bearing angle plane with the ellipsoid. If, further, the jammer altitude can be estimated, the intersection of the aforementioned line with the plane of constant altitude determines jammer position. Again an assumed altitude is used making this technique unreliable. In addition, a cooperating or noncooperating ground surveillance radar is required.
Two other techniques that apply to airborne emitters bear mentioning. The first is ground imaging of the area directly underneath the emitter by comparing the ground-reflected intercept energy with the directly-received intercepted energy which is used as a reference. The jammer position is then determined by comparing the image obtained from the intercept with aerial photographs or images obtained by other means. This requires appreciable time which is rarely, if ever, available in a combat situation.
Another technique is called PROSE, for Passive Ranging On Scanning Emitters. This technique is applicable to ranging on an enemy AWACS-like emission: that is, the emission from a scanning radar in place of the jammers or jammer-like sources considered above. In this case, the enemy radar scan rate can be measured rather accurately using the average interval between main lobe intercepts determined over a long time base. If then two intercept antennas are provided on the friendly surveillance aircraft, and the time delay between corresponding main beam intercepts on these two antennas is calculated rather accurately, the parallax angle, or angle of rotation of the enemy scanning radar, between the intercepts at the two antennas can be calculated. Then, knowing the distance between the two intercept antennas, the range to the enemy scanning radar from the friendly surveillance aircraft can be calculated. Although this approach is a parallax-like technique, it requires a large aperture for the scanning radar antenna and a long baseline, neither of which may be available.
A typical passive ranging scenario is shown in FIG. 7. As noted above, the traditional problem is to obtain the range of an unfriendly emitting target using angle of arrival information obtained via a directional receiver onboard a friendly vehicle. FIG. 7 shows a first flight path 701 of a jamming aircraft, a second flight path 702 of a surveillance aircraft, first, second and third segment of the surveillance flight path designated by drawing numeral 703, 704 and 705, respectively, a corresponding first, second and third segment of the jammer flight path designated by drawing numerals 709, 710 and 711, respectively, and strobes connecting the respective centers of the first, second and third segments of the two flight paths, designated by drawing numerals 706, 707 and 708. To aid in distinguishing the three different flight path segments and their respective strobes, a dotted line has been used for the first, a dashed line for the second and a solid line for the third.
Accordingly, in FIG. 7, three jammer bearing angles are shown: the first dotted, the second dashed, and the third in solid. The corresponding segments of the jammer and surveillance flight paths are shown in dotted, dashed and solid line coding. If the jammer had been fixed, the three strobes would intersect in a point when projected back from the known locations of the surveillance aircraft at the time the strobes were received. The back projection of the strobes would intersect at the estimated position of the fixed jammer. The case that is shown, however, is for a moving jammer. The conventional approach is based upon the assumption that the tangential component of jammer velocity is constant. When jammer tangential acceleration is much smaller than the radar tangential acceleration component (as when the surveillance aircraft is flown in an arc), and the time intervals are equal, range is estimated by locating the position in range (which is along the X-axis, 712 in this Figure) for which the strobes are equally spaced and correctly ordered in cross range (which is along the Y-axis 715 in this Figure). The effect of radial component of jammer motion is small, as it causes only a slight error in the estimated range of the jammer. This approach, however, depends strongly upon the identification of each strobe with a particular unique jammer.
In FIG. 8, a second jammer has been added. This jammer is moving on a path 801 in an opposite direction to the first 701 and is located at a different range from the surveillance aircraft. Again, the new jammer strobes are coded in accordance with the position of the radar and jammer at the time of the observation. Thus, the first strobe on each jammer is coded by dotted lines 706 and 806, the second by dashed lines 707 and 807, and the third by solid lines 708 and 808, as in the previous Figure.
This Figure shows that if the radar operator is unable to identify a particular jamming strobe with a particular jammer (as, for example, when both jammers are wide band noise of identical parameters), unique jammer ranging may not be possible. For example, if the wrong dashed strobe is assumed to be identified with the second jammer, its range would be incorrectly estimated as being about half-way between the two actual jammer positions.
The problem then is how to estimate jammer positions unambiguously without the need for identification of a particular strobe with a particular jammer.
An approach to the solution of the problem involves the use of a modified form of a technique that has been in use recently in the field of modern X-ray technology. This technique, called Computer Augmented Tomography (CAT) X-ray, has been well developed for mapping continuously distributed emitters or absorbers using angle measurements only. The CAT approach, unfortunately, works only with a fixed object (emitter or absorber). However, a review of this technique as it presently is applied to fixed objects is necessary to understand any modifications which would make it applicable to the solution. In order to do this, a review of least means (LMS) ranging will be presented first.
FIG. 9 shows a typical radar flight path 901 in dashed lines and three angle of arrival strobes 902, 903 and 904 in dotted lines that would be obtained on a single fixed jammer 907 (circle). LMS ranging defines one of the three strobes as a reference and calculates the perpendicular distance from the reference strobe to each of the other two at a particular value of range (X). As an example, the two short dotted lines 905 and 906 are the perpendicular distances that would be obtained for a value of X somewhat shorter than the true range of the jammer. In LMS ranging, the length of the two short dotted lines are squared and then summed, and X is varied until a minimum value is found for the sum. This condition would obviously be obtained when the test value of X was equal to the range of the jammer. In fact, even if the strobe positions were in error due to errors in the angle of arrival measuring system or in the supposedly known position of the surveillance aircraft at the time of the measurements, the LMS approach would give the optimum estimate of the range of a single jammer.
When a second jammer is added, however, unless the strobes can be kept separate--that is, identified uniquely with a particular jammer, LMS ranging would give a single jammer range estimate which is midway between the two jammers.
The example is shown in FIG. 10. The second jammer 1007 has angle of arrival strobes 1002, 1003 and 1004 in solid lines, and the perpendicular distances that correspond to the LMS estimate of range at the ends of the callout lines 1005 and 1006 formed of alternate dots and long dashes. This approach obviously does not work for multiple jammers.
CAT ranging, on the other hand, as it is applied to X-raying, may be immediately applied to fixed jammers. The technique works by generating for each jamming strobe a weighted back projection which is weighted most strongly (that is, brightest on a display system) at the measured angle of arrival of the jammer strobe and which fades in intensity corresponding to the potential accuracy of the angle of arrival information. As an example, if the angle of arrival sensing system is known to be accurate to about 5 degrees, the back projection would be weighted to have half intensity 5 degrees away from the measured angle of arrival of the received jammer signal.
Jammer positions would now be estimated by overlaying back projections corresponding to each of the three jammer strobe positions of the previous illustration.
FIG. 11 shows what would happen if the strobes were presented on a cathode-ray tube display with one jammer where the back projections were unweighted. Notice that the three strobes 1102, 1103 and 1104 on the display are brightest in the area designated by drawing numeral 1105 where the strobes overlay one another which is only at the estimated position of the jammer. These strobes are of equal, moderate intensity. Where they all cross, the intensities add, and a bright white area 1105 is produced. In order to simulate this effect, the strobes are shaded with the exception of the crossover area 1105. If the back projections had been weighted, an even better estimate of the position of the jammer would have been obtained. Of course, for a single jammer, LMS ranging could have been used to obtain a comparable estimate of jammer position.
However, when the second jammer is added, it can be seen from FIG. 12 that the jamming strobes 1102, 1103, 1104, 1202 and 1204 are automatically identified with the correct jammers. That is, 1102, 1103 and 1104 are associated with the jammer at 1105, and strobes 1103,1202 and 1204 are associated with the jammer at 1205. The final display is brightest only at the positions of the two jammers 1105 and 1205. Other ambiguous positions that correspond to potential jammers form an image that is considerably less bright than the true position. Of course, the more back projections there are, the more accurate this technique is. In the original CAT X-ray technique, a continuous set of back projections from all aspect angles is used to generate an excellent image of a completely distributed object.
As noted, the use of the state of the art CAT X-ray technique provides a solution for stationary jammers, but unfortunately, in its present form fails to provide a solution for moving jammers, which a most common problem, encountered regularly with airborne jammers.